The use of radar to determine range and velocity of objects in an environment is important in a number of applications including automotive radar and gesture detection. A radar system typically transmits radio signals and listens for the reflection of the radio signals from objects in the environment. By comparing the transmitted radio signals with the received radio signals, a radar system can determine the distance to an object. Using multiple transmissions, the velocity of an object can be determined. Using multiple transmitters and receivers, the location (angle) of an object can also be determined.
There are several types of signals used in different types of radar systems. One type of radar signal is known as a frequency-modulated continuous waveform (FMCW). In an FMCW radar system, the transmitter of the radar system sends a continuous signal in which the frequency of the signal varies. This is sometimes called a chirp radar system. Mixing (multiplying) the reflected wave from an object with a replica of the transmitted signal results in a CW signal with a frequency that represents the distance between the radar transmitter/receiver and the object. By sweeping up in frequency and then down in frequency, the Doppler frequency can also be determined.
Another type of radar signal is known as a phase-modulated continuous waveform (PMCW). For this type of radio signal, the phase of the transmitted signal is modulated according to a certain pattern or code, sometimes called the spreading code, known at the PMCW radar receiver. The transmitted signal is phase modulated by mixing a baseband signal (e.g., with two values +1 and −1) with a local oscillator to generate a transmitted signal with a phase that is changing corresponding to the baseband signal (e.g., +1 corresponding to a phase of 0 radians and −1 corresponding to a phase of π radians). For a single transmitter, a sequence of phase values that form the code or spreading code that has good autocorrelation properties is required so that ghost objects are minimized. The rate at which the phase is modulated determines the bandwidth of the transmitted signal and is called the chip rate.
In a PMCW radar system, the receiver performs correlations of the received signal with time-delayed versions of the transmitted signal and looks for peaks in the correlation. The time-delay of the transmitted signal that yields peaks in the correlation corresponds to the delay of the transmitted signal when reflected off an object. The distance to the object is found from that delay and the speed of light.
A PMCW radar will receive not only the reflected signals from the transmitter of the PMCW radar. A PMCW radar operating in the presence of an FMCW radar will also receive the signals from the FMCW radar. The signal from the FMCW radar can significantly affect the performance of a conventional PMCW radar system. Potentially, these FMCW signals can be much stronger than the reflected signals from the PMCW radar. This may cause the radar system's estimated range, velocity and direction to be significantly in error.